Green Party: Caroline Lucas and Jonathan Bartley elected as co-leaders

They saw off competition from five others to succeed Natalie Bennett, who is stepping down after four years.

Ms Lucas, the Greens' only MP, was leader of the party between 2008 and 2012 while Mr Bartley is the party's work and pensions spokesman.

The two said the joint election showed the party was "not bound by tradition".

Their joint ticket took 13,570 - 88% - of the 15,467 votes cast.

The announcement was made at the party's autumn conference in Birmingham, at which Amelia Womack was also elected deputy leader.

Ms Lucas thanked outgoing leader Ms Bennett and said that, under her leadership, the party had "reached more people than ever before".

How the job-share arrangement will work in practice has yet to be made clear - the party says co-leadership is "a first in Westminster politics".

But, to some laughter from party members, Mr Bartley said: "We stand here, more united as a party with two leaders than others are with one" and invited those who wanted "more than divisions and uncertainty" to join.

The co-leaders said their joint election showed the "the power of working together and the importance of striking a healthy balance between work and family and other commitments".

Ms Lucas told the BBC later that the job-share sent a political message about "doing politics differently" but it had also been "a very practical decision" as Mr Bartley has a disabled son which made it harder to work on politics "24/7", while she could spend more time in her Brighton Pavilion constituency.

In an acceptance speech delivered together, Ms Lucas and Mr Bartley called for the Greens to form "progressive alliances" with like-minded parties.

Mr Bartley said the current first-past-the-post voting system for general elections was "redundant" and it was wrong that while more than a million people in England and Wales voted Green in 2015, it only had one MP.

"We are resolute in wanting to explore the potential for progressive alliances with other parties that will deliver fair votes and will deliver more elected Greens that ever before," he said.

This has been an unusual kind of leadership race, five candidates competing against one pair: Caroline Lucas and Jonathan Bartley running as a job-share.

As the hundreds of delegates, keen with anticipation, flooded into Birmingham University's Great Hall to hear the announcement of the new leader and the victory speech, the game was up: There were two podiums ready on the stage.

Ms Lucas and Mr Bartley are presenting the job-share as an example of how they want politics to work: co-operation and unity, as against what they see as the rancour, bitterness and division left in the aftermath of the EU referendum result.

But it is a certain irony for a party that, nearly a decade ago, abandoned its system of having two leaders - one male, one female - or "principal speakers" as they were then known.

One of those who was most vigorous in campaigning for the single leader system at the time - the party's now co-leader, Caroline Lucas.

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Ms Lucas has previously floated the idea of a loose electoral pact with other parties on the left of British politics, including Labour, which would see them give Green candidates a clear run in constituencies where they were best placed to challenge the Conservatives and vice versa.

Any such arrangements would have to be approved at a local level, Mr Bartley said and would not be imposed.

The BBC's political correspondent Tom Bateman said many Green activists were sympathetic to the idea but did not believe it was realistic given Labour's opposition.

Much of the speech touched on the UK's vote to leave the EU and its aftermath.

Ms Lucas accused Leave campaigners of lying to the British people during the referendum and "then running away" with no plan for the aftermath.

She said environmental protections, workers rights, guarantees for EU citizens already living in the UK and a "culture of free movement" should be at the heart of the UK's Brexit settlement - the terms of which she said should be put to a second referendum.

And she won applause from the party faithful for her comments on "the greatest threat to our security today - the accelerating climate crisis" saying fossil fuels should be left "where they belong... in the ground". "No fracking, no nuclear, no compromise," she added, to big cheers from the audience.

Until 2008, when Ms Lucas was elected for the first time, the party did not have a single leader but relied on two principal speakers.

The other candidates in the election were former election candidate Simon Cross, long-serving member Clive Lord, film-maker David Malone, parish councillor Martie Warin and David Williams, who leads the Green group on Oxfordshire County Council.